The field of the invention relates to oscillators and more particularly to temperature controlled crystal oscillators.
Temperature controlled crystal oscillators (TCXOs) are generally known. Such devices are typically constructed in the form of a crystal and a controlling chip. Within the chip, a set of switchable capacitors and a feedback amplifier form a tank circuit that oscillates at a frequency determined by the number of capacitors switched into the tank circuit.
A temperature sensor is typically provided within the chip for sensing a temperature in the environs of the crystal. Based upon the temperature, a controller switches capacitors into and out of the tank circuit based upon a performance criteria of the tank circuit which is typically stored in a lookup table within the TCXO chip.
While prior art TCXOs work well, their structure and mode of operation is complex. In order to meet a frequency drift specification, a lookup table of frequency response versus temperature is typically stored within the controller. While the use of a lookup table is effective, the lookup table typically requires a relatively large amount of memory. Accordingly, a need exists for a better method of storing temperature drift characteristics.
A method and apparatus are provided for generating a control signal for compensating a temperature controlled crystal oscillator system for changes in an ambient temperature. The method includes the steps of providing a set of second-order coefficients relating ambient temperature to frequency drift over at least a portion of an operating temperature range of the temperature controlled crystal oscillator system and calculating a control signal for compensating the temperature controlled crystal oscillator system for the frequency drift due to the ambient temperature based upon the set of second-order coefficients.